This invention relates to thin film high voltage electrographic writing heads for recording information on a recording medium and, in particular, to improvements in the control of the phenomenon known as "flare" or "flaring" that occurs upon electrode discharge in electrographic writing processes.
It is known in the electrographic writing head art to employ for electrographic writing a plurality of spatially disposed conductive electrode lines deposited on an insulating substrate which terminate in a nib or stylus. The nibs may be of any metal suitably formed using for photolithographic and electroforming techniques such as copper, nickel or tungsten, or may be polysilicon formed on a silicon or ceramic substrate. Examples of such electrographic writing head structures are disclosed in U.S. Pat. Nos. 4,356,501 and 4,415,403. These thin film electrographic writing head structures also have included driving logic and circuitry integrally fabricated upon the same head substrate, such as the multiplexed driving circuit, high voltage and low voltage thin film transistors and accompanying address and data bus lines. Low voltage address lines operate to selectively address nibs or groups of nibs for discharging by applying a high voltage to the stylus via its connected high voltage thin film transistor. Such an arrangement is shown in U.S. Pat. No. 4,588,997. Pulse forming circuit arrangements may include a R-C network between a voltage source and the nibs for the purpose of providing a lower address voltages to the nibs and facilitating the supply of voltage to the nibs to cause a sufficient discharge for latent image writing. Examples of such networks are shown in U.S. Pat. Nos. 4,030,107, 4,359,753 and 4,466,020.
One of the problems encountered in this technology is that the discharge from the nibs is not always uniform so that the latent image spots created on the recording medium nonuniform in shape and enlarged or irregular in size compared to other latent image spots. This phenomenon is known in the art as "fare" or "flaring". Flare is detrimental to the quality of printed or plotted images on the recording medium because the spot sizes formed on the recording medium on discharge of the nibs are not uniform and flare out in an irregular pattern. Also, arcing across nibs to the recording medium further causes such enlargement and destructive disfiguration of the uniformity of spot size. To prevent flaring from occurring, limiting resistors have been placed in the driving logic or in the electrode lead lines leading to the nibs to limit the flow of current to the nibs and prevent such arcing and spot size irregularity. Examples of resistance that particularly function in this manner are disclosed in Russian patent publication No. 611,173 and U.S. Pat. No. 4,415,403, which respectively illustrate limiting resistors 3 and 82 in electrode lead lines to stylus or nib 1 and 88.
However, the problem of flaring still prevails in the art in spite of the utilization of such limiting resistors. Flaring still occurs and spot sizes, while being more uniform in size, still remain with ragged edges and nonuniform size.